Rising fuel costs and increasingly stringent environmental regulations such as carbon taxes are driving the development of aircraft propulsion systems with improved fuel efficiency and reduced carbon consumption. One propulsion system or propulsor configuration which is known to provide improved fuel efficiency is the open fan propulsor. The open fan propulsor may be configured similar to a ducted turbofan engine commonly used in commercial aircraft with the exception that an open fan propulsor may include counter-rotating rotors located forward of or on an exterior of the engine nacelle in contrast to a ducted turbofan engine which includes one or more fans located in the interior of the engine nacelle.
One drawback associated with open fan propulsors is their high noise output. Studies have shown that open fan propulsors produce unacceptably high levels of noise that would undesirably impact communities near airports. In addition, the high noise levels of open fan propulsors may impact communities located under the flight path of the aircraft during climb out of the aircraft. Furthermore, aircraft having open fan propulsors are subject to increasingly strict noise requirements imposed by governing bodies such as the Federal Aviation Administration (FAA) and the International Civil Aviation Organization (ICAO). For example, the FAA administers a noise certification regulation that is harmonized with ICAO and which sets limits on the amount of noise that an aircraft may produce during takeoff and landing.
For noise certification, the FAA requires the measurement of takeoff noise and landing noise to verify that such noise is below defined limits. Takeoff noise includes sideline noise and flyover noise. Sideline noise is measured at a set lateral distance from the runway centerline during takeoff of the aircraft. Flyover noise is measured at a set distance from a downstream end of the runway under the flight path as the aircraft flies over the measurement location. For landing, the FAA requires the measurement of approach noise which is measured from a position underneath the aircraft glide slope as the aircraft approaches the runway threshold. However, approach noise is generally, but not exclusively, the result of the influence of air flowing over and around the airframe components such as the landing gear and wing flaps. Engine noise contributes a comparable portion to the cumulative approach noise of an aircraft due to the relatively low power settings of the aircraft engines during approach.
In efforts to reduce sideline and flyover noise, studies have been undertaken to identify engine-level noise-reducing technologies that could be applied to open fan propulsors. The identified engine technologies were primarily directed toward the propulsor configuration and arrangement of the counter-rotating forward and aft rotors of the open fan propulsor. For example, one of the noise-reducing technologies proposes increased spacing between the forward and aft rotors. Other noise-reducing technologies include blade-cropping to reduce the overall diameter of one of the rotor blades discs, and altering the blade shape and configuration to mitigate rotor-borne blade noise. Unfortunately, each one of the noise-reducing technologies also results in a decrease in thrust and fuel efficiency of the open fan propulsor.
As can be seen, there exists a need in the art for an aircraft arrangement for reducing the noise produced by an open fan aircraft while maintaining the fuel efficiency benefits provided by an open fan propulsor. Ideally, the reduction in noise is achieved without introducing non-aerodynamic surfaces on the aircraft and without increasing the size or quantity of existing aerodynamic surfaces for noise blockage purposes.